Orthopedic (bone) implants are commonly used to replace some or all of a patient's joint, such as a hip, knee, shoulder or elbow, where deterioration of or damage to the joint due to aging, illness, injury or trauma is present. These implants are designed to accommodate the normal movements and stresses associated with such joints and to provide increased mobility and relief from pain. Orthopedic implants are most typically constructed of metal, polymeric material or some combination thereof.
Proper fixation of the implant within the bone is an extremely important aspect of joint replacement inasmuch as long-term implant success is dependent, in large part, on the implant remaining relatively stationary within the implantation site. Proper and reliable fixation of the implant allows bone tissue to grow into and around the implant to establish long-term fixation of the implant within the bone.
Typically, initial stabilization of the implant is achieved by applying bone cement (e.g., polymethylmethacrylate (PMMA)) during surgery. Bone cement acts as filler between the bone and the implant. However, long term stabilization of the implant is often achieved and dependent on osseointegration (i.e., bone tissue ingrowth into the implant) of the implant with the surrounding bone of the patient. Accordingly, orthopedic implants are commonly provided with an outer bone contacting surface adapted for allowing bone ingrowth. For example, implants may be provided with a porous bone contacting surface which would allow bone tissue ingrowth over time. This porous bone contacting surface may be provided during manufacture of the implant or incorporated into the implant in a post-molding treatment.
As noted above, while proper fixation of the orthopedic implant is an important aspect of the joint replacement surgery, post-surgical considerations also play a role in achieving successful longer term joint replacement. Natural bone requires that it be cyclically stressed to survive and remain strong as bone that is not subjected to normal stresses and loads will lose bone density and weaken. Some orthopedic implants that include a metal component may result in an implant which has an elastic modulus that is much greater than that of cortical bone. Therefore, the implant is much stiffer than the bone in which it is to be implanted. A large difference in the elastic modulus of the implant on the one hand and the bone on the other may cause the implant (rather than the bone) to support and absorb most of the loads imparted on a joint, leaving the bone virtually unloaded or unstressed. This phenomenon, which is commonly referred to as stress shielding, can result in the formation of debris around the implantation site, bone loss and/or bone resorption.
Accordingly, orthopedic implants with metal have been developed that reduce or otherwise limit the effects of stress shielding. One example of such an implant is the EPOCH® hip prosthesis available from Zimmer, Inc., of Warsaw, Ind. The EPOCH® hip stem has a porous surface that allows for fixation of the implant in the femur through natural bone tissue ingrowth. In addition, the EPOCH® hip stem is made of materials that closely match the stiffness of bone. The EPOCH® device includes a metal core made of a cobalt-chromium-molybdenum alloy, a middle portion made of a high strength thermoplastic material and an outer surface layer made of a titanium fiber metal mesh. The fiber metal mesh provides a porous bone contacting surface that allows for bone growth into the implant while the overall construction of the hip stem provides a stiffness comparable to the stiffness of a normal femur, thereby reducing the potential for stress shielding.
Metallic porous bone contacting surfaces may also include pads or beads embedded in the body of the implant. Examples of implants that include such porous, metallic surface layers, including fiber metal mesh of the type described above, are disclosed in U.S. Pat. Nos. 5,219,363, 5,236,457, 5,443,512 and 6,740,186 all of which are herein incorporated by reference in their entireties.
While systems such as the EPOCH® hip stem have proven to be commercially successful and have effectively addressed bone fixation and stress shielding, use of non-metallic materials in the bone contacting surfaces of orthopedic implants has also been considered. Thermoplastics, thermoplastic composites, ceramics and other non-metallic materials may be suited to match the stiffness of bone and thereby limit the effects of stress shielding. These materials may also be provided with a porous structure to allow for bone ingrowth and aid in implant fixation. In addition, some of these materials are often advantageously shaped by economical molding processes such as injection molding or compression molding. Examples of such implants and methods are described below.